1. Field of the Invention
The present invention relates to a handover control method, a mobile station and a communication control apparatus used for the handover control method in a mobile communication system including a communication apparatus having a function of a base station, the mobile station and the communication control apparatus which controls connection of communication channels between the communication apparatus and the mobile station.
2. Description of the Related Art
In a conventional mobile communication system, the mobile station always receives perch channels from a base station which is a communication apparatus with which the mobile station communicates (base station which is a transmit/receive partner of user data, such base station will be called a communication partner) and base stations surrounding the base station of the communication partner, and measures received powers of the perch channels. Then, when a received power of a perch channel from a surrounding base station is larger than a received power of the base station of the communication partner by more than a predetermined value, the mobile station performs handover to the surrounding base station according to control by a base station control apparatus which is a communication control apparatus which controls communication channels between the mobile station and the base station. As a result, the connection destination is switched. This handover operation is called hard handover.
In a mobile communication system using code-division multiple access (CDMA), the mobile station can connect to a plurality of base stations simultaneously. Thus, being controlled by the base station control apparatus, the mobile station releases connection to a base station which is originally connected after establishing connection to a base station of a handover destination. This handover operation is called soft handover. However, the soft handover is performed in the same way as the hard handover. That is, in the soft handover, the mobile station measures received powers of perch channels sent from the base stations of the communication partner and the surrounding base stations. Then, when a received power of a perch channel from a surrounding base station is larger than a received power of the base station of the communication partner by more than a predetermined value, the mobile station performs handover to the surrounding base station controlled a base station control apparatus which is a communication control apparatus which controls communication channels between the mobile station and the base station so that the connection destination is switched However, according to the above-mentioned conventional method, handover is always performed when the received power of the perch channel from the surrounding base station is larger by more than the predetermined value, even when the received power of the perch channel from the base station of the communication partner is large enough, that is, even when handover is unnecessary. Therefore, the conventional method is not necessarily a good method from the view point of efficient use of wireless resources. In addition, there is a problem in that large load is imposed to the base station control apparatus which controls the handover and the mobile station.
In addition, according to the conventional method, the wireless resource of the base station of the handover destination is not considered at all. Therefore, when a request of handover is sent to a base station, there is a case in which the request is rejected due to lack of wireless resource so that communication is disconnected forcefully.